Desalination by reverse osmosis is achieved by pumping a feed stream of saline water at an elevated working pressure into a pressure resistant vessel containing semipermeable membranes. Purified product water of greatly reduced salinity permeates across the membranes into low pressure collection channels if the working pressure exceeds the feed stream osmotic pressure. Excess working pressure above the feed stream osmotic pressure is required to produce sufficient product water flux across membranes of reasonable surface area, and also to ensure sufficient dilution of the small but finite salt diffusion through the membrane which always exists where there is a concentration gradient across such membranes.
While some of the feed stream permeates through the membranes, the balance becomes increasingly concentrated with salt rejected by the membranes. In a continuous reverse osmosis process, a concentrate stream must be exhausted from the vessel to prevent excessive salt accumulation. In commonly available manually operated reverse osmosis apparatus the concentrate stream is directed to the back side of the single pump piston through a combination of directional valves and mechanical dwell and the energy due to pressurization is used to assist the operator in pumping. Furthermore, the pressurized concentrate is then exhausted to ambient, thus controlling concentration polarization.
Concentration polarization in the feed stream is the tendency for a concentration gradient to develop in the feed stream with high salt concentration on the membrane face during reverse osmosis. This tendency results from the bulk transport of saline water toward the membrane face and the accumulation of salt in the boundary layer as less saline water permeates through the membrane, balanced by diffusion of salt back out of the boundary layer. Concentration polarization is detrimental especially with feed streams of high osmotic pressure such as sea water, because the membrane sees a higher concentration which raises the effective osmotic pressure. When concentration polarization occurs, working pressure for given product flux must be increased, product salinity will be increased and membrane life may be impaired.
Manually operated reverse osmosis systems are typically designed to reduce concentration polarization effects by forced convection through the membrane array. Forced convection may be provided by circulating a low ratio of the permeate stream through suitably configured feed channels between membrane faces. However, it is generally not desirable to bypass a portion of the permeate stream to reduce concentration polarization.
It is desirable to provide a continuous feed stream circulation through the membrane array, because even momentary stagnation of flow may cause severe concentration polarization.
Operation at low ratios of permeate flow to concentrate flow is also generally favorable to the reduction of concentration polarization effects.